Such a clamping apparatus has been known from DE-A 197 16 874. It comprises one or more tensioners, which can be positioned on a clamping table and with which the workpiece, e.g., body parts of vehicles, can be clamped. The workpiece can then be welded or machined in another way by an industrial robot or the like. The tensioners are of a multipart design and comprise a foot part, a middle part and a head part, which are guided adjustably relative to another along at least one axis. A positioning apparatus with vertical adjustment apparatus and a fixing device are present for the mutual adjustment. The vertical adjustment apparatus comprises one or more adjusting screws between the parts of the tensioner. The foot part may also be fastened to the clamping table via a spacer placed under it. The fixing device comprises an elongated hole guide with two clamping screws. Moreover, the positioning apparatus may also comprise an angle adjustment apparatus with an additional pivot bearing between the parts of the tensioner. As an alternative, it is also possible to use a bent middle part. The lower adjusting screw for the vertical adjustment apparatus is reserved for the supplier and is used to set a fixed basic length of the clamping unit specified by the user. Only the upper adjusting screw between the middle part and the head part is then accessible to the customer and user. He can then perform a fine adjustment within a narrow range by apparatus of this adjusting screw. The tensioners are accurately positioned according to the user's specifications and hold the workpiece in the specified position. Inaccurate workpieces (pilot run) as well as warping during machining may make it necessary to adjust the clamping points. This adjustment can be carried out by the customer with the prior-art tensioner only with the upper adjusting screw and in a range limited by the screw. The inaccuracies inherently existing in adjusting screws must be accepted here. The tensioner as a whole can be adjusted vertically continuously by apparatus of the adjusting screws.
WO 96/38265 shows another clamping unit, which comprises a bracket, a clamping frame, a centering piece and a support that can be fastened thereto. This clamping unit has a fixed, predetermined length, which is determined by the length of the support cut as needed. The support may assume different angular positions in relation to the foot-side bracket, which can be achieved by dividing the clamping frame and the centering piece into mutually independent components and by apparatus of correspondingly shaped connection surfaces.
U.S. Pat. No. 2,497,943 discloses another clamping unit for sewing machines, typewriters, electric motors or the like for repair purposes. It has a pivotable bracket with clamping jaws for this purpose, which can be adjusted by apparatus of a spindle.
Clamping pallets for workpieces on machine tools or gaging machines have been known from DE-A 195 36 341, DE-A 42 43 370 and DE-A 38 01 813. The tensioners have clamping claws, which are vertically adjustable by apparatus of adjusting screws, toothed plates or the like. Finally, a clamping unit with a plurality of clamping parts that are angularly adjustable in relation to one another has been known from GB-A 1 466 434.